Originally Posted by FBinNY

I assume this is a steel bike. If so, this is a pretty bad dent, but not likely to be catastrophic, or lead to sudden failure. Interestingly, the cheaper the frame the longer it'll last before it finally (if ever) lets go. That's because high alloys are somewhat less ductile, and more prone to eventual fatigue.

Go ahead and ride it as is, or if you like, you can camouflage it with some body filler and some trim. Wears ago when I was in retail we had adhesive foil tape in Olympic colors, and could often make repairs look like intentional trim.

As you ride it, bare of filled and covered, keep an eye open for any crack propagation, which will probably start at the edges of the dent. This could happen in months, years or decades. Steel crack propagation is fairly slow, so if you keep an eye on it you should have plenty of warning of imminent failure.